The present invention relates to load indicating members and load indicating fasteners and further relates to methods and apparatuses for making and using load indicating members and load indicating fastening members. More particularly, the present invention relates to a method of measuring a load in an ultrasonic load indicating member, a load measuring device using the method of measuring the load, a fastener tightening tool for tightening an ultrasonic load indicating fastener member using the method of measuring, an ultrasonic load indicating member and an ultrasonic load indicating fastener for use in conjunction with the method of measuring, a method making the ultrasonic load indicating fastener, a method of tightening the ultrasonic load indicating fastener, and a transducer for instrumenting the load indicating member.
In many operations, it is desirable to determine the amount of longitudinal load experienced by a longitudinally stressed member. This information is particularly useful when the longitudinally stressed member is a fastener since the measurement of the longitudinal stress provides a verification of the existence of a proper joint.
Many prior techniques have been developed to indicate the amount of longitudinal stress experienced by a fastener providing a load indicating feature on the fastener itself. This is usually done by interconnecting one end of an elongated member, such as a pin, to a portion of the fastener prior to the installation of the fastener.
An example of this type of load indicating member is depicted in FIG. 1 of the drawing. The elongated member 10 extends parallel to the fastener 12 and is unaffected by the elastic deformation of the fastener in response to longitudinal stress. The free end 14 of the elongated member 10 therefore provides a reference for indicating the elongation of the fastener 12 in response to the longitudinal stress Typically, the elongated member 10 is a pin received with in an elongated bore 16 disposed longitudinally of the fastener 12 and extended from the head 18 of the fastener and partially through the shank 20 of the fastener. One end 22 of the pin 10 is interconnected with the shank 20 of the fastener 12 at the base of the bore 16 by means, for example, of adhesives, threads, or an interference fit. The various prior load indicating members and load indicating fasteners of this type differed greatly in structure as well as in the methods and apparatuses with which they were used to provide an indication of the elongation of the load indicating member or fastener. Examples of this type of fastener are disclosed in U.S. Pat. Nos. 3,812,758 issued May 28, 1974 to Robert B. Bessler, Jr.; 3,823,639 issued July 16, 1974 to Theodore Liber; 2,503,141 issued Apr. 4, 1950 to Albert R. Stone; 3,943,819 issued Mar. 16, 1976 to Charles S. Charron; 2,600,029 issued June 10, 1952 to Albert R. Stone, 3,908,508 issued Sept. 30, 1975 to William J. Payne; 3,987,66 issued Oct. 26, 1976 to Charles H. Popenoe; and 4,144,428 issued Sept. 19, 1978 to Charles H. Popenoe; as well as in commonly assigned U.S. patent application Ser. No. 670,260 filed 11/13/84, now U.S. Pat. No. 4,676,109.
While each of the various pin-type load indicating members, and load measuring devices described above provides its own advantages in terms of accuracy, ease of manufacture, or ease of reading, they are still expensive to manufacture, since they each require extensive modifications and the addition of a centrally located pin-member to the load indicating member. As a result such load indicating members are only selectively used in practice, either where there is a specific immediate diagnostic need or a serious recognized safety hazard involved. These members are simply too expensive for routine use in assemblies which may only occasionally benefit from such monitoring.
An alternate approach to measuring the elongation of a member or fastener is to use an ultrasonic measurement device. Typically, this is done, as shown in FIG. 2 of the drawing, by removably interconnecting an ultrasonic transducer 24 to one end of the member being measured, typically to the head 26 of a fastener 28. In order to obtain a reliable indication, the head of the bolt must be ground extremely flat and a reliable ultrasonic transmission media must b applied to the head of the bolt. The transducer must be properly positioned on the bolt and held in position while the measurements are being taken. Examples of techniques and apparatuses using this method are disclosed in U.S. Pat. Nos. 3,306,100 issued Feb. 28, 1967 to Wilheim et al.; 3,307,393 issued Mar. 7, 1967 to Kessler; 3,308,476 issued Mar. 7, 1967 to Kleesattel; 3,759,090 issued Sept. 18, 1973 to McFaul et al.; 3,812,709 issued May 28, 1974 to Benson et al.; 3,822,587 issued July 9, 1974 to Makino et al.; 4,014,208 issued Mar. 29, 1977 to Moore et al.; 4,062,227 issued Dec. 13, 1977 to Heyman; 4,117,731 issued Oct. 3, 1978 to Heyman; 4,363,242 issued Dec. 14, 1982 to Heyman; 4,402,222 issued Sept. 6, 1983 to Olson et al.; 4,413,518 issued Nov. 8, 1983 to Jones and 4,471,657 issued Sept. 18, 1984 to Voris et al.
The patents teach the notion of combining the measuring device with a tightening tool so that the information gained from measuring the elongation of the bolt can be used determining when to shut off the tightening tool or, alternatively, monitor the tightening process to determine whether a proper joint has been formed. Examples of such tightening tools are disclosed in U.S. Pat. Nos. 3,969,960 issued July 20, 1976 to Pagano, 3,969,810 issued July 20, 1976 to Pagano.
While the above listed methods and apparatuses can provide reliable information about a fastener and a joint, they are in very limited use. This is mainly because the bolt must be carefully manufactured and must be easily accessible to the instrumentation. Thus, ultrasonic tension measurement is recognized as a highly accurate laboratory tightening method for calibration application testing and for tightening very critical joints. It is replacing strain gage bolts in several calibration and critical quality control applications. However, the practical difficulties associated with taking ultrasonic tension measurements described above have prevented its application as a general assembly tightening strategy.
Some attempts have been made to combine the advantages of the pin-type load indicating members, described previously, with the ultrasonic elongation measurement device described above by incorporating a piezoelectric or other ultrasonic sensor into the member itself. Examples of such members are disclosed, for example, in U.S. Pat. No. 4,127,788 issued Nov. 28, 1978 to Daugherty and U.S. Pat. No. 4,294,122 issued Oct. 13, 1981 to Couchman. Each of these disclosures provide an instrumented load bearing fastener which has been modified to incorporate a stress indicating feature. However, like the pin-type fasteners described previously, these instrumented fasteners are greatly modified in order to accept large and complicated ultrasonic sensing devices. They are therefore prohibitably expensive for wide spread use.
Examples of additional prior patents known to the applicant which teach or claim ultrasonic piezoelectric or alternate methods are:
______________________________________ U.S. Pat. No. Issue Date Inventor ______________________________________ 3,201,977 8/24/65 Kutsay 3,306,100 2/28/67 Wilhelm, Lyndhurst, Kliever 3,307,393 3/7/67 Kessler 3,308,476 3/7/67 Kleesattel 3,541,844 11/24/70 Stover 3,650,016 3/21/72 McMaster 3,759,090 9/18/73 McFaul 3,810,385 5/14/74 McFaul 3,812,709 5/28/74 Benson 3,822,587 7/9/74 Makino 3,918,294 11/11/75 Makino 3,924,444 12/9/75 Heyman 3,930,405 1/6/76 Renken 3,969,810 7/20/76 Pagano 3,969,960 7/20/76 Pagano 3,975,948 8/24/76 Makino 4,014,208 3/29/77 Moore 4,015,464 4/5/77 Miller 4,062,227 12/13/77 Heyman 4,117,731 10/3/78 Heyman 4,121,467 10/24/78 Gerhart 4,127,788 11/28/78 Daugherty Re. 30,183 1/8/80 Popenoe 4,294,122 10/13/81 Couchman 4,363,242 12/14/82 Heyman 4,402,222 9/6/83 Olson 4,413,518 11/8/83 Jones 4,423,634 1/3/84 Audenard 4,445,360 5/1/84 Treder 4,471,657 9/18/84 Voris 4,569,229 2/11/85 de Halleux 4,567,766 2/4/86 Seiferling 4,584,676 4/22/86 Newman 4,601,207 7/22/86 Steblay 4,602,511 7/29/86 Holt ______________________________________
Very few actual products have resulted from these developments and their use has generally been limited to laboratory work and expensive and critical installations due to the difficulty in maintaining reliable coupling during tightening, the expense and complexity of the required equipment, and the strict control required over fastener materials and properties.
What is needed, therefore, is a low cost ultrasonic transducer permanently attached to a fastener in an inexpensive manner to provide accurate tightening information on a mass production basis. Such an ultrasonic load indicating member would permit easy interconnection with measuring or assembly tool devices and avoid the problems encountered with prior ultrasonic measuring devices in attempting to obtain a reliable acoustical coupling.